Generally, as illustrated in FIGS. 1 and 2, a cooling device for a power semiconductor device has a structure in which a heatsink 104 is brought into pressure contact with power semiconductor modules 101 by tightening bolts through an insulating material 102 and a thermal interface material 103 such as grease and a thermally conductive adhesive. Fans 105 are installed for forced air cooling to blow air in a direction along heat radiation fins 104B of the heatsink 104.
Heat generated in the power semiconductor device during an operation of a power converter is released into an ambient environment (atmosphere) from the power semiconductor modules 101 through the thermal interface material 103, which is a contact boundary, and a base 104A and heat radiation fins 104B of the heatsink 104.
However, in the case of the aforementioned forced-air cooling type cooling device for the power semiconductor device, a heat-transfer coefficient (several tens W/m2K) of the heatsink 104 which serves as a heat radiation part is low relative to a heat density (several hundred thousands W/m2) of the power semiconductor device, and, in order to keep the temperature difference within acceptable variation (several tens ° C.), the radiation surface area needed to be expanded several hundred times the heated surface area.
In a process of expanding the surface area, there are factors which prevent heat radiation, including thermal conductivity resistance (thermal resistance due to heat conduction in a solid), contact thermal resistance (thermal resistance due to contact between solids), spreading thermal resistance (thermal resistance due to heat transfer from a heat generating component to the heatsink 104 while spreading at an angle of 45 degrees), a fm efficiency (a correction factor for a non-uniform temperature of the entire heat radiation fins 104B), a heatsink efficiency (a correction factor for a non-uniform temperature of incoming and outgoing air). Therefore, the volume of the heatsink 104 has been much larger than that of the power semiconductor module 101.
A power exchanger illustrated in FIG. 1 constructed of a large IGBT module and a cooling device will be described as an example of a conventional cooling device of a power semiconductor device. If a heat loss of the power semiconductor device is 2000 W and an acceptable junction temperature is 125° C. (ambient temperature of 40° C.), applying a large-sized caulking joint type heatsink 104 (W 330 mm×L 300 mm×H 110 mm) with forced air cooling fans 105 mounted thereon is one of reasonable solutions. At this time, since the thermal resistance of the heatsink 104 is 0.028 K/W and the volume of the same is 10890 cm3, the volume thermal resistance is 305 cm3 K/W (the performance index of the heatsink).
In order to reduce the volume of the heatsink 104, cooling means for efficient heat radiation is necessary. As one of such means, there is a heat-pipe type or vapor type cooler which transports heat by using latent heat and heat transfer due to evaporation condensation of a cooling medium, and a volume of a heatsink can be reduced by approximately a half to a third. This heatsink is widely used as a cooling device for an electric vehicle as well.
Another example of the means is a water-cooling type cooler which transports heat by means of forced circulation of a cooling medium using a pump. Yet another example is a cooler in which a micro channel is constructed right next to a heat generating component to reduce thermal conductivity resistance and the radiation surface area is expanded to reduce the surface thermal resistance to a cooling medium, so a flow rate of coolable heat is increased, thus enabling to cool a heat generating component with a high heat generation density. Also, there is a cooler which reduces a surface thermal resistance by using an impinging jet to obtain a similar effect.
However, in the conventional devices, even though a size of a heat receiving block can be reduced, a separate gas-liquid heat exchanger is needed to release heat into the ambient environment (atmosphere). So, the volume of the water-cooling type cooing device including peripherals (a driving pump, a tube, and the like) becomes equal to or larger than that of a heat-pipe or vapor type cooling device.
As explained above, in the conventional cooling devices of the power semiconductor device, a heat transport mechanism realized by circulation of a cooling medium is required. Therefore, the entire costs of a heatsink including a heat receiving block, a heat transport mechanism, and heat radiation fins have been increased. Further, in the conventional cooling devices of the power semiconductor device, since the heat receiving block and the heat radiation fins can be separated from each other, there is high layout flexibility. However, the volume of the entire cooling device including the heat receiving block, the heat transport mechanism, and the heat radiation fins is reduced by approximately a half or a third, which is not so small. It is also a problem that measures need to be taken to address possible issues such as freezing and leakage of a cooling medium.